It has been disclosed by our group that the polymerization of propylene in contact with stereospecific catalysts supported on anhydrous magnesium dihalides has important advantages.
According to one such method, propylene is polymerized in contact with a catalytic system prepared by mixing
A. an alkyl aluminum compound complexed with an electron-donor compound, particularly an organic ester such as ethyl anisate, ethyl benzoate, etc. with PA1 B. a titanium tri- or tetrahalide in which the halogen is Cl, Br or I, and preferably Ti Cl.sub.4, or a complex of the Ti halide with an electron-donor compound, supported on anhydrous Mg dihalide, e.g., Mg Cl.sub.2.
The preparation of such supported catalysts has been described.
The polymerization is carried out in liquid phase either in the presence or absence of solvents such as aliphatic or cycloaliphatic hydrocarbons, e.g., n-pentane, n-hexane, n-heptane or cyclohexane.
Alkyl aluminum compounds useful in preparing catalyst-forming component ( A ) include triethyl aluminum, tri- isobutyl aluminum, tri-n-octyl aluminum and diethyl aluminum monochloride. Preferably, catalystforming component ( A ) is triethyl aluminum complexed with the electrondonor.
Catalyst-forming component ( B ) is preferably a supported catalytic complex of the type Ti Cl.sub.4. ethyl benzoate or of the type Ti Cl.sub.4.2 ethyl benzoate.
In the organometallic component ( A ) consisting of the aluminum alkyl partially complexed with the electron donor, the molar ratio aluminum alkyl/electron donor may vary from 1 to 20. Best results are obtained when the complex-forming electron donor is ethyl anisate; good results are also obtained when it is ethyl benzoate. The preferred organometallic component ( A ) is an aluminum triethyl/ethyl anisate complex in a molar ratio of 3.
Polymerization of propylene in the presence of the supported catalysts results in very high yields of polypropylene, from 50,000 to 200,000 and more grams of polymer per gram of titanium used, and thus results in a polypropylene having a low content of catalytic residues. In some instances, especially when a polypropylene having a high isotacticity index is desired, in which event the polymer yield is lower, it is convenient to reduce the content of catalytic residues by subjecting the polypropylene to suitable after-treatments.